Bubble bursts;Letter

15th May 1998 at 01:00
I very much enjoyed Ted Wragg's article entitled "Bursting with ideas" in TES Primary magazine on April 24. I shall certainly be trying out his bubble-making ideas this summer with my children.

However, I think some of the physics was wrong. The colours on bubbles are caused by interference, not refraction. Also the bubbles do not sink because water drains to the bottom. They sink because their weight is greater than the upthrust they experience (the redistribution of water in the bubble does not change its weight).

We experience upthrust when we float in water: it is equal and opposite to our weight. The draining of water to the bottom of the bubble changes the thickness of the soap film, and it is this that causes the coloured patterns to move across the bubble. The thinning of the soap bubble eventually causes it to appear black and burst.

Ted says: "At this early age the physical universe can be incredibly exciting." I still think it is!

John Williams Head of science New Hall School Chelmsford

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